When introducing experimental extras I forgot to update this check. That resulted in experimental extras running through the steps normally reserved for non-extra natives: macro and constant expansion, validation, and minification. This causes problems since per the linked bug minification is buggy.

R=yangguo@chromium.org, jochen@chromium.org
BUG=v8:4064
LOG=N

Review URL: https://codereview.chromium.org/1400253002

Cr-Commit-Position: refs/heads/master@{#31270}